Commercially-available sunscreen products are often formulated as emulsions. Emulsions are two-phased, sometimes three-phased, systems of materials that are immiscible in each other. Two-phased emulsions are commonly classified as water-in-oil or oil-in-water, where the latter-mentioned phase is the continuous, outer part of the emulsion. The continuous phase need not comprise the majority by weight of the emulsion composition. By their very nature, emulsion systems are unstable; in time, all emulsions will separate into their constituent parts. Emulsifiers are therefore added to these systems to maintain stability. By “emulsion stability” is meant a system in which the oil and water phases do not separate within a period of at least 3 years.
Sunscreen products absorb a certain percentage of light over a specified spectrum. The SPF listed on sunscreen products is related to this percentage and is intended to communicate the amount of erythemal UVR attenuation. Theoretically, the numeric SPF value tells the user that he or she is protected X times longer than without sunscreen where X is the labeled SPF. For example, an SPF 30 product would, theoretically, absorb 96.7% of erythemal UVR and allow about 3.3% of unattenuated erythemal UV light to reach the skin. The user of such an SPF 30 product would conclude that he or she could stay out in the sun 30 times longer than without the sunscreen. The actual amount of protection from a particular sunscreen product depends on factors including the skin type of the user, the amount applied and frequency of re-application, time of day and season, amount of sunscreen active(s) absorbed by the skin, amount of sunscreen active(s) removed from the skin (e.g., by perspiration, swimming), and photodegradation of the active(s) themselves.
The most common effect of UVR exposure is erythema, or sunburn. In 1987, the Commision Internationale de l'Éclarage adopted a reference standard, Minimal Erythemal Dose (“MED”), which indicates the minimum dose of UVR that will produce a noticeable reddening of human skin that has not been previously exposed to UVR. MED is related to skin type.
Among sunscreen emulsion products, those of the oil-in-water type are more common. This, in part, is due to aesthetic considerations; oil-in-water emulsions have a pleasant cooling feel upon application, similar to that of water. Oil-in-water emulsions are also generally considerably less expensive to manufacture than water-in-oil emulsions of a similar SPF. Oil-in-water emulsions also suffer from a well-known disadvantage—the water-soluble emulsifiers used in oil-in-water emulsions are detergents which remove oil from the skin, including oil-soluble sunscreens contained in the oil phase of sunscreen emulsion products. As a consequence, the efficacy of an oil-in-water sunscreen emulsion in providing protection from damaging ultra-violet radiation (“UVR”) is diminished as soon as the emulsion comes into contact with water (e.g., from moisture, perspiration or swimming.)
In contrast, water-in-oil sunscreen emulsions also referred to as “invert emulsions”—use water-insoluble emulsifiers. Because these emulsifiers are not detergents, water-in-oil sunscreen emulsions confer a degree of water resistance; they are less easily removed from the skin. Consequently, given equal amounts of sunscreen actives, water-in-oil sunscreen emulsions typically have a higher SPF than oil-in-water sunscreens. However, it is well-known to those of ordinary skill in the art that it is difficult to formulate stable invert emulsion systems. Water-in-oil emulsions, for example, often exhibit viscosity changes prior to phase separation, a phenomenon described as creaming. An emulsion that exhibits creaming is not homogenous from top to bottom, and thus is less efficacious from the perspective of consistently delivering the same, or substantially the same, desired level active ingredients with each application.
Silicone emulsifiers were developed to improve the stability of invert emulsion systems and thus delay separation of water-in-oil emulsions into their constituent oil and water phases. Silicone emulsifiers typically have less than twenty percent water-soluble groups (e.g., polyoxyalkylene), with the remainder of the emulsifier molecule being comprised of alkyl and silicone moieties. One silicone emulsifier used in water-in-oil emulsions is cetyl dimethicone copolyol, described in now-expired U.S. Pat. No. 4,698,178, and sold under the trade-name Abil EM-90 by Goldschmidt.
The use of an effective emulsifier is particularly important in sunscreens, which are often used many months, sometimes years, after production. The use of Abil EM-90 in water-in-oil sunscreen formulations is described in Examples 3, 4 and 5 of U.S. Pat. No. 6,936,241 (Col. 16, line 35-Column 17, line 35). These examples disclose the combination of at least one organic sunscreen and at least one hydrophobically-treated inorganic sunscreen. Emulsions made with Abil EM-90, however, have been observed to stratify (e.g., into layers of varying solubilities) when stored at cold temperatures. This is not observed in emulsions made with a different class of alkyl dimethicone copolyol, one in which water-soluble, alkyl-soluble and silicone-soluble groups are in specific ratios to each other. Silfsurf J-208-812, sold by Siltech LLC, is a lauryl dimethicone copolyol illustrative of the latter class of alkyl dimethicone copolyols. Prior to the current invention, the importance of the relationship between emulsifier selection and sunscreen efficiency was not realized. We have surprisingly found that the selection of an emulsifier with particular properties is critical to achieving a high level of sunscreen efficiency, and thereby decreasing the amount of sunscreen needed to achieve a desired SPF.
Emulsion stability, while important, is but one of several considerations in formulating an effective sunscreen composition. Equally, if not more important, is the selection of a combination of sunscreen actives that is photostable throughout the UVB and UVA spectra (i.e., from 290-400 nm) under conditions of actual use (i.e., in natural sunlight as opposed to under artificial spectra generated by a solar simulator).
Many sunscreen actives are not photostable. It is well-known to those of ordinary skill that certain organic sunscreens are oxidatively, or photooxidatively, unstable. For example, the photoinstability of avobenzone is described in U.S. Pat. Nos. 5,576,354 and 5,993,789. Pending U.S. patent application Ser. No. 10/887,464, now Publication No. 2005/0025727, describes the photoinstability of octinoxate, octisalate and homosalate. Examples 3, 4 and 5 in U.S. Pat. No. 6,936,241 each teach a water-in-oil sunscreen emulsion comprising cetyl dimethicone copolyol (Abil EM-90) and a photolabile sunscreen—octisalate. One of these examples comprises a second photolabile organic sunscreen, avobenzone. A technical brochure for Silfsurf J-208-812 discloses a water-in-oil sunscreen emulsion containing three sunscreens, two of which—octisalate and octyl methoxycinnamate—are not photostable. The weight percentage of three sunscreen actives in the emulsion made with Silfsurf J-208-812 is 17.5 and produce an expected SPF of less than about 30; it is not a high SE product according to the present invention.
Photoinstability is problematic for several reasons. First, photodegradation of sunscreen actives results in lesser amounts of effective sunscreen being present over the exposure time. A photolabile sunscreen blend therefore provides a lesser degree of photoprotection than is indicated by labeled SPF. Second, photodegradation reactions generate free radicals, which are associated with adverse health consequences, including damage to DNA and other cellular molecules. This is even more of a concern since breakdown products of photolabile sunscreens have been reported to penetrate the skin and be absorbed systemically.
U.S. Patent Application Publication No. 2004/0047818 discloses a sunscreen composition comprising avobenzone, less than 1% octocrylene (weight/weight), and a diester or polyester of naphthalene dicarboxylic acid. None of the sunscreen compositions taught in Publication No. 2004/0047818 comprise the triplet combination of avobenzone, octocrylene and oxybenzone alone, with no substantial amount of other photodegradable sunscreen actives, or with substantially no other sunscreen active present.
U.S. Patent Application Publication No. 2004/0166070 discloses non-pilling UV-photoprotecting alcoholic sunscreen gels comprising acrylates/C12-C22 alkylmethacrylate copolymer and an effective amount of at least one UV-A and/or UV-B screening agent where the screening agent comprises avobenzone, octocrylene, oxybenzone and/or octyl salicylate. Lower monohydric alcohols, most commonly ethanol and isopropanol, are used in the alcohol sunscreen gel compositions disclosed in this application. Example 1 discloses an ethanolic sunscreen gel containing acrylates/C12-C22 alkylmethacrylate copolymer in combination with four sunscreens (avobenzone, octocrylene, oxybenzone and octyl salicylate). Embodiments of the present invention do not contain one or more elements of the sunscreen composition disclosed in Publication No. 2004/0166070 including, but not limited to, the following: (i) the emulsion system of the present invention may be essentially free of lower monohydric alcohols or free of lower monohydric alcohols; (ii) the present invention may be essentially free of, or free of, acrylates/C12-C22 alkylmethacrylate copolymer in an effective non-pilling amount used to thicken (i.e., gel) a low molecular weight alcohol (i.e., C1-C4 alcohol).
U.S. Patent Application Publication No. 2005/0013781 discloses photoprotective compositions comprising one or more sunscreens and one or more “optimizing agents” which are defined as diols, alcohols, glycols, polyhydric alcohols as well as derivatives or combinations thereof that optimize SPF, Protection Factor A, Boots Star Rating, polarity, critical wavelength or photostability (or any combinations of the foregoing) of the oil phase, water phase, both phases of the composition, or the final sunscreen formulation. Polyhydric alcohols, in particular certain glycols, are disclosed in this application as “optimizing agents”. Publication No. 2005/0013781 also discloses over thirty sunscreens, including avobenzone, octocrylene and oxybenzone. Table 21 discloses a composition comprising five sunscreens (homosalate, octyl salicylate, oxybenzone, octocrylene and avobenzone) in combination with two optimizing agents (1,2 octanediol and neopentyl glycol). The photostable water-in-oil emulsions of the present invention are, in preferred embodiments, substantially free of substantial amounts of “optimizing agents”, preferably substantially free of “optimizing agents”, and more preferably free of “optimizing agents.”
Where sunscreen emulsions have a lower SE—due, for example, to the type of emulsion (e.g., oil-in-water vs. water-in-oil), emulsion instability, photoinstability of sunscreens in the emulsion, or a combination of the above—consumers can be mislead. Labeled SPF may not be indicative of the photoprotection actually provided, causing consumers to believe, mistakenly, that they can safely stay out in sun for longer periods of time than that for which the sunscreen actually provides protection. The adverse consequences of overexposure to UVR when wearing a less efficient sunscreen product (e.g., one not having an SE of at least 2) are not limited to sunburn but include potentially more serious long-term health effects, including exposure to harmful breakdown products of unstable sunscreen actives. Accordingly, there remains a long-felt but unmet need for a sunscreen emulsion that provides a high sunscreen efficacy, one with an SE value of at least 2, and preferably at least 3. Preferably, such a sunscreen emulsion would contain a photostable combination of sunscreen actives as well as resistant to removal by perspiration or from water activities. It would also have good long-term emulsion storage stability over a wide temperature range for extended periods of time. This need is met by the present invention.
Prior to the present invention, water-in-oil sunscreen emulsions using a photostable sunscreen blend (one containing avobenzone, octocrylene and oxybenzone) were observed to stratify resulting in less than optimal ratios of the sunscreens to each other and, consequently, a lower SE than that of the present invention. Surprisingly, and unexpectedly, the combination of photostable sunscreens and alkyl dimethicone copolyol, where the alkyl-soluble, water-soluble and silicone-soluble groups are in specified ratios to each other, results in sunscreen compositions having a greater SE than had been previously available. The high SE water-in-oil emulsions of the present invention do not stratify over a wide range of temperatures or over extended periods of time and provide consumers with a level of predictable, photostable, photoprotection heretofore not achieved.